1. Field of the Invention
This invention relates generally to subsea completion systems. In particular, the invention concerns a subsea completion system which may be considered a hybrid of conventional xmas tree (CXT) and horizontal xmas tree (HXT) arrangements. More specifically, this invention relates to a marine riser/tubing hanger/tubing spool arrangement with the capability of passing production tubing and a large number of electric and hydraulic lines within a relatively small diameter.
This invention also relates to a method and arrangement whereby both "reduced bore" ("slimbore") and conventional BOP/marine riser systems may be interfaced both to the tubing spool and the xmas tree, such that the BOP stack need not be retrieved in order that the xmas tree may be installed, and so that the xmas tree need not be deployed with or interfaced at all by a conventional workover/intervention riser, if this is not desired.
2. Background and Objects of the Invention
The invention described below originates from an objective to provide a subsea completion system that is capable of being installed and serviced using a marine riser and BOP stack, especially those of substantially reduced size and weight as compared to conventional systems. One objective is to replace a conventional 19" nominal bore marine riser and associated 183/4" nominal bore BOP stack with a smaller bore diameter system, for example in the range between 14" and 11" for the marine riser and BOP stack. Preferably the internal diameter of the BOP stack is under 12". If the riser bore diameter is under 12", it will require only 40% of the volume of fluids to fill in comparison to 19" nominal conventional systems. The smaller riser/BOP stack and the resulting reduced fluids volume requirements result in a significant advantage for the operator in the form of weight and cost savings for the riser, fluids, fluid storage facilities, etc. These factors combine to increase available "deck loading" capacity and deck storage space for any rig using the arrangement of the invention and facilitates operations in deeper water as compared to arrangements currently available.
At the same time, it is desirable to accommodate a large number of electric (E) and hydraulic (H) conduits through the tubing hanger. A currently available tubing hanger typical of those provided throughout the subsea completion industry can accommodate a production bore, an annulus bore, and up to one electric (1E) plus five hydraulic (5H) conduits. An important objective of the invention is to provide a new system to accommodate production tubing and provide annulus communication, and to provide a tubing hanger that can accommodate (ideally) as many as 2E plus 7H independent conduits. The requirement for the large number of E and H conduits results from the desire to accommodate downhole "smart wells" hardware (smart wells have down-hole devices such as sliding sleeves, enhanced sensing and control systems, etc., which require conduits to the surface for their control).
It is also an object of the invention to provide a subsea system that obviates the need for a conventional, and costly, "open sea" capable workover/intervention riser. The object is to provide a system which allows well access via a BOP stack/marine riser system on top of a subsea xmas tree. Such a system is advantageous, especially for deep water applications, where the xmas tree can be installed without first having to retrieve and subsequently re-run the BOP stack. Another important object of the invention is to provide a system which allows future intervention using a BOP stack/marine riser or a more conventional workover/intervention riser.